Laser wavelength control unit with piezoelectric driver

Abstract

An electric discharge laser with fast wavelength correction. Fast wavelength correction equipment includes at least one piezoelectric drive and a fast wavelength measurement system and fast feedback response times. In a preferred embodiment, equipment is provided to control wavelength on a slow time frame of several milliseconds, on a intermediate time from of about one to five milliseconds and on a very fast time frame of a few microseconds. Preferred techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver for tuning the laser wavelength using a tuning mirror. A preferred control technique is described (utilizing a very fast wavelength monitor) to provide the slow and intermediate wavelength control with the combination of a stepper motor and a piezoelectric driver. Very fast wavelength control is provided with a piezoelectric load cell in combination with the piezoelectric driver. Preferred embodiments provide (1) fast feedback control based on wavelength measurements, (2) fast vibration control, (3) active damping using the load cell and an active damping module, (4) transient inversion using feed forward algorithms based on historical burst data. A preferred embodiment adapts the feed forward algorithms to current conditions. Another preferred embodiment measures tuning mirror position to permit wavelength pretuning and active wavelength tuning.

Description

Background of the InventionLight Sources For Integrated Circuit FabricationAn important use of gas discharge excimer lasers is to provide high quality light sources for integrated circuit fabrication. These light sources are used by stepper machines and scanner machines for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the stepper and scanner machines are designed for a particular laser beam with a narrow band of wavelengths. The output beam of the excimer laser is typically comprised of a very narrow band of wavelengths distributed about a "central" wavelength (referred to as the "line center" wavelength) approximately in a gausian distribution. For krypton fluoride (KrF) and argon fluoride (ArF) excimer lasers the output beam is "line narrowed" to produce the desired narrow band of wavelengths. The laser center line wavelength may drift over time and, thus, a feedback network is typically employed to det...

AI Technical Summary

Problems solved by technology

The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light.

However, they have not been very effective in controlling wavelengths over short time periods of about 3 to 30 milliseconds or very short time periods of about 1-3 milliseconds or less.

Conventional wavelength correction techniques do not adequately correct these large and sudden wavelength shifts near the beginning of each burst of pulses.

Conventional prior art wavelength correction techniques also are not adequate to correct the small very rapidly occurring (high frequency) wavelength shifts that occur throughout the burst.

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